Caprolactam can be produced from three hydrocarbon feedstocks: cyclohexane, phenol, and toluene. Approximately 68% of the world's caprolactam capacity is produced from cyclohexane, 31% from phenol, and 1% from toluene. All of the cyclohexane and phenol-based production proceeds via the formation of cyclohexanone oxime. In 94% of the cyclohexane and phenol-based caprolactam capacity, the formation of this oxime requires an ammonia oxidation step.
In the processes involving ammonia oxidation, caprolactam production from cyclohexane or phenol can be broken down into the following steps:
Oxidation of cyclohexane or hydrogenation of phenol, to synthesize cyclohexanone; PA1 Oxidation of ammonia to form nitric oxide, followed by various reaction steps to form a hydroxylamine salt; PA1 Synthesis of cyclohexanone oxime by reaction of cyclohexanone and the hydroxylamine salt; and PA1 Treatment of the cyclohexanone oxime with sulfuric acid followed by neutralization with aqueous ammonia to form caprolactam. PA1 (a) reacting air with ammonia gas in an ammonia conversion zone to produce nitric oxide; PA1 (b) oxidizing at least a portion of the nitric oxide to nitrogen dioxide to produce an NO.sub.x -rich process gas stream; PA1 (c) reactively absorbing the NO.sub.x -rich gas stream with phosphoric acid containing solution in an absorption zone to form nitrate ions; PA1 (d) contacting the nitrate ions with air in a degassing zone to produce a nitrate-rich aqueous process stream; PA1 (e) reducing the nitrate-rich aqueous stream with hydrogen in the presence of phosphoric acid to produce hydroxylammonium phosphate; PA1 (f) oximating the hydroxylammonium phosphate with cyclohexanone to produce cyclohexanone oxime; and PA1 (g) converting the cyclohexanone oxime to caprolactam.
One such method for producing caprolactam is the DSM-HPO (Dutch State Mines-Hydroxylammonium Phosphate-Oxime) process, also known as the Stamicarbon process. Such process is disclosed, for example, in Weissermel and Arp, Industrial Organic Chemistry (VCH Verlagsgesellschaft mbH 1993), pp. 249-258. In the DSM-HPO process, hydroxylammonium phosphate (NH.sub.3 OH.H.sub.2 PO.sub.4) is reacted with cyclohexanone in toluene solvent to synthesize the oxime.
The hydroxylammonium phosphate is synthesized in the DSM-HPO process in the following manner:
Catalytic air oxidation of ammonia to form nitric oxide: EQU 4 NH.sub.3+5 O.sub.2.fwdarw.4 NO+6 H.sub.2 O (I)
Continued oxidation of nitric oxide to form nitrogen dioxide, among other nitrogen oxides: EQU NO+1/2 O.sub.2.fwdarw.NO.sub.2 (II)
Reactive absorption of nitrogen dioxide in a buffered aqueous phosphoric acid solution to form nitrate ions: EQU 3 NO.sub.2 +H.sub.2 O.fwdarw.2 HNO.sub.3 +NO (III) EQU HNO.sub.3 +H.sub.2 PO.sub.4.fwdarw.NO.sub.3 +H.sub.3 PO.sub.4 (IV)
Catalytic hydrogenation of nitrate ions to form hydroxylammonium phosphate: EQU NO.sub.3.sup.- +2 H.sub.3 PO.sub.4 +3 H.sub.2.fwdarw.NH.sub.3 OH.H.sub.2 PO.sub.4 +H.sub.2 PO.sub.4.sup.- +2 H.sub.2 O (V)
Oximating the cyclohexanone with hydroxylammonium phosphate to produce cyclohexanone oxime: EQU C.sub.6 H.sub.10 O+NH.sub.3 OH.H.sub.2 PO.sub.4.fwdarw.C.sub.6 H.sub.11 NO+H.sub.3 PO.sub.4 +H.sub.2 O (VI)
The process for forming hydroxylammonium phosphate in the DSM-HPO process is shown in the flow sheet depicted in FIG. 1 of the attached drawing. As shown therein, an air stream 3 is initially compressed in a compressor 10, introduced as a "primary" air stream through feed line 12 into admixture with a gaseous ammonia stream 1, and thereafter fed to a catalytic ammonia converter 20. Typically, 100% ammonia conversion and 95% selectivity to NO are achieved in that reaction. Upon exiting the converter, some of the NO is further oxidized to NO.sub.2 to form an NO.sub.x -rich process gas stream 15. Some of the NO.sub.2 in the NO.sub.x -rich process stream 15 dimerizes to form N.sub.2 O.sub.4.
The NO.sub.x -rich process gas stream 15 is contacted countercurrently with an aqueous inorganic acid stream 37 in a trayed absorption tower 40. In the conventional DSM-HPO process, a "secondary" air stream 11 is added into a degasser 50 in amounts of from 5 to 20 volume % of the total air flow to the system. The secondary air stream 11 becomes laden with nitric oxide and the resulting nitric oxide laden air stream 17 is added to the base of the absorption tower 40. A nitrate-rich liquid stream 13 exiting the absorption tower 40 is routed to the degasser 50, and an NO, containing vent gas 5 exits the absorption tower.
The vent gas 5 exiting the absorption tower 40 must normally be properly regulated to minimize the emission of NO,. An increase in production of hydroxylammonium phosphate typically results in a corresponding increase in NO.sub.x emission in the vent gas 5.
The aqueous inorganic acid stream 37 added to the top of the absorption tower 40 contains a mixture of water, phosphoric acid (H.sub.3 PO.sub.4), ammonium nitrate (NH.sub.4 NO.sub.3), and monoammonium phosphoric acid (NH.sub.4 H.sub.2 PO.sub.4). The acid stream 37 is continuously cycled from the oximator train (consisting of an oximator 70, oxime extractor 80, and a hydrocarbon stripper 90) to the hydroxylamine train (consisting of the absorption tower 40, degasser 50, and a nitrate hydrogenator 60). Nitric oxides in the NO.sub.x -rich process gas stream 15 reactively absorb in the phosphoric acid solution in the absorption tower 40 to form nitrate ions.
The nitrate-rich liquid stream 13 exiting the absorption tower 40 is passed through the degasser 50, where it is contacted countercurrently with secondary air 11 entering the degasser 50. The secondary air 11 removes unreacted nitric oxides from the nitrate-rich liquid stream 13. The nitric oxide-containing air stream 17 exiting the degasser 50 is routed to the absorption tower 40.
A nitrate-rich liquid stream 19 exiting the degasser 50 is combined with an aqueous inorganic acid stream 21 from the oximator train, and the combination 31 fed to the nitrate hydrogenator 60. A hydrogen stream 7 is also added to the nitrate hydrogenator 60. Nitrate ions are reduced with hydrogen in the nitrate hydrogenator 60 over a palladium catalyst to form hydroxylammonium phosphate. An aqueous stream of hydroxylammonium phosphate, phosphoric acid, ammonium nitrate, and monoammonium phosphoric acid 23 exits the nitrate hydrogenator 60.
The hydroxylammonium phosphate containing aqueous stream 23 then reacts with a stream of cyclohexanone in toluene solvent 25 in the oximator 70 to produce cyclohexanone oxime. An oxime-toluene stream 9 exits the oximator 70 and is processed into caprolactam. An aqueous stream 27 also exits the oximator 70, and is routed to a oxime extractor 80 which removes entrained oxime 39, and adds it to the stream of cyclohexanone in toluene solvent 25. An aqueous stream 29 exiting the oxime extractor 80 is routed to a hydrocarbon stripper 90 where entrained cyclohexanone and toluene 33 are removed and added to the stream of cyclohexanone in toluene solvent 25, which is routed to the oximator 70. Thus, the entrained oxime 39 obtained in the oxime extractor 80 and the cyclohexanone-toluene 33 obtained in the hydrocarbon stripper 90 are returned to the oximator 70. The aqueous stream 35 leaving the hydrocarbon stripper 90 is routed back to the hydroxylamine train, where a portion 21 is distributed to the nitrate hydrogenator 60 and a portion 37 is distributed to the absorption tower 40. Typically, about 90% of aqueous stream 35 is routed to stream 21, and about 10% routed to stream 37.
In view of the strict environmental regulation of NO.sub.x emissions, the quantity of NO.sub.x in the vent gas 5 cannot be increased. Accordingly, any increased hydroxylammonium phosphate production (and subsequent caprolactam production) must be obtained without any increase in NO.sub.x emissions. This can be accomplished by increasing the amount of air and ammonia fed to the process while increasing the plant size, e.g., the size of the absorption tower 40 and air compressor 10. However, such an increase in equipment capacity requires substantial capital investment.
There is therefore a need for the development of improved techniques in the DSM-HPO process for producing caprolactam, by which increased amounts of hydroxylammonium phosphate and, consequently, caprolactam can be produced without large capital investment, and without increasing NO.sub.x emissions.